Spectators of events, such as sports events often wish to purchase a souvenir as a memoir of the event they have attended. Typical souvenirs include items which are indirectly related to the event. For example, hats, shirts, pins or any other item bearing the symbol and name of the sports team are souvenirs not related to the particular event in which a person actually takes part as a spectator.
However, it is not practical to record images of an event in progress and simultaneously or within a short time thereafter produce a print therefrom. Images generally require a large amount of storage space and preferably the process of capturing the image and then transferring it onwards for printing is best achieved by compressing (and later de-compressing) the images.
Representing an image as digital data (typically in a bitmap) requires a large amount of storage space. For example, if the image is of the A3 size (11.5 in.times.17 in) at 300 dots per inch (dpi) resolution, the image will have 17.4 million pixels. If the image is a color image stored at 32 bits/pixel, the image will require 66 Mbytes of storage. To print a stored image requires a printer having a large amount of Random Access Memory (RAM), which increases the cost and size of memory circuits. To reduce the amount of space required for storage, numerous compression techniques have been developed.
However, for the compression technique to be effective, it has to be "lossless", that is the decompressed image must strongly resemble the original image. The Joint Photographic Experts Group (JPEG) has a still-image coding scheme which provides a virtually lossless compression ratio of 7:1 for "bitmap" images (for example, images produced from photographs). A compression ratio of 2:1 is usually lossless.
Unfortunately, for artificial images such as are produced in a pre-press environment, the JPEG compression technique provides an unacceptably high amount of loss. Such artificial images, or "pages", are common to many publications and are combinations of bitmap images, text and vectors. The JPEG compression technique cannot compress the text and shape elements of the `page` without significant losses in the quality of the areas surrounding the lines forming the text and shape elements. JPEG is not suitable for printers requiring good image quality.
Other compression techniques are known. For example, the page can be run-length encoded, such as via the Linework format of Scitex Corporation Ltd. of Herzlia, Israel. In run-length encoding, each sequence of neighboring pixels which have the same color pixels are compressed into a color value and a length of the run of similar colored pixels. Run-length encoding provides varied compression ratios which depend on the amount of color changes in the image. For images, such as text, which have long lengths of similar colored pixels, run-length encoding provides 1:20 compression ratios for a whole page. However, for continuous tone images, such as those scanned from a photograph, run-length encoding provides terrible compression ratios of up to 2:1 (i.e. the "compressed" image is twice as large as the uncompressed one).
The article "Implementation of Image Compression for Printers", by K. Oka and M. Onishi, Color Hard Copy and Graphic Arts, SPIE Vol. 1670, 1992, describes a compression technique for artificial images. Their compression technique utilizes a block truncation coding (BTC) technique which codes a 4.times.4 pixel block of page data. An original image with 24 bits/pixel for each color component is compressed into 9 bits/pixel, a compression ratio of 3:8. The BTC technique has the advantage of being able to compress both computer graphics and text images. However, the BTC technique does not produce a small compressed file. For certain applications, further compression is still required.